As you can see, the url index looks like it should be a better index for the query, with two columns instead of one. The shortcut I often use to diagnose issues like this is EXPLAIN EXTENDED, followed by SHOW WARNINGS. The resulting warning message is often revealing:

The easiest solution in this case was to change the ’m’ table’s definition, because it is a scratch table used as part of an import process.

I'm Baron Schwartz, the founder and CEO of VividCortex. I am the author of High
Performance MySQL and lots of open-source software for performance analysis, monitoring, and system administration.
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